On the hemnet link, if you sarch for less then 20qm it finds 12. If you search eg. for 30-40qm it finds more then 600. Doesn't look like <20qm would be some kind of standard.
Another example - Paris, 87 ads for condos 0-20m^2 [0], 104 for 30-40m^2 [1] on pap.fr, a popular site. Similar numbers for rentals ("location").
[0] http://www.pap.fr/annonce/vente-appartements-paris-75-g439-j...
[1] http://www.pap.fr/annonce/vente-appartements-paris-75-g439-e...